This site will calculate the declination for you.

The declination changes according to your location (both longitude and latitude). It also changes yearly, so what's printed on your map may be out of date, especially if it's an old map. It's a good idea to look up the declination where you are going and write it on your map.

Current declination at the west end of the Columbia River Gorge (where I live) is 16 degrees 3 minutes E, moving west at the rate of 9 minutes per year. If the map you're using is more than 7 years old, the declination marked on it is already a degree off.